Digital echoes: open bus behavior on the compact Macintosh
6 days ago
- #vintage computing
- #hardware behavior
- #emulation
- Explains open bus behavior in vintage Macintosh systems like the 128K, 512K, Plus, SE, and Classic.
- Describes how the Motorola 68000 CPU interacts with memory and devices via an address/data bus.
- Details the concept of tri-state signals and parasitic capacitance affecting data reads from unassigned memory regions.
- Illustrates how the Microbug debugger can reveal residual data from previous bus transactions.
- Explains the role of the MC68000's prefetch queue in observing specific data patterns like '6116'.
- Discusses the necessity of cycle-accurate emulation to replicate open bus behavior correctly.
- Provides examples of software affected by open bus behavior, including The Print Shop 1.0 and Animation Toolkit 1.0.
- Highlights a bug in Animation Toolkit 1.0 that causes it to crash on a Macintosh Plus with 4MB RAM due to improper memory handling.
- Emphasizes the importance of accurate emulation, even if it replicates crashes or errors present in real hardware.
- References technical manuals and contributions from community members for verification and testing.